This invention relates to novel polyimides, to end-capped polyimides, and to the polyamic acid and ester intermediates thereof. It also relates to their use as adhesives or molded articles. It further relates to novel primary aromatic diamines for use in the preparation of the polymers.
Polyimides are synthetic organic resins characterized by repeating imide linkages in the polymer chain which may or may not be end-capped with polymerizable or inert (i.e. non-polymerizable) chemical groups. They are available in both linear and crosslinked forms and are noted for their outstanding chemical and physical properties, particularly their high temperature oxidative stability and strength. In addition to their use as adhesives and molded articles, they may be used as precured films and fibers, curable enamels, and laminating resins.
The polyimides, especially the preferred aromatic polyimides, are extremely difficult to process due to their insolubility and extremely high softening points. Early attempts to decrease the softening temperature usually involved the substitution of aliphatic segments into the otherwise aromatic polymers, but this generally resulted in an accompanying decrease in thermooxidative stability. In order to overcome these disadvantages and to improve their processability and mechanical properties, it has been found advantageous to introduce flexibilizing moieties, such as bipyridyl, sulfone, alkylene, or preferably ether or thio bridges, into the polymer chain and thus provide a product with improved flexibility and impact strength. Crosslinking moieties have also been introduced into the polymers by the use of some portion of a more highly functional amine (e.g. triamine) or by the introduction of carboxyl or hydroxyl groups.
A further disadvantage exists if the polyamic acid or ester intermediates will not be used immediately. The solutions, preferably concentrated, must be stored at low temperatures and protected from moisture to prevent premature imidization (i.e. ring closure). In addition, during curing to the fully or partially imidized resin, as appreciable amount of a volatile by-product (e.g. water, alcohol, or hydrohalides depending upon the starting tetracarboxylic acid compound) is formed. This leads to the formation of voids when the resin is used as an adhesive between metal substrates or for forming molded articles. In order to overcome these disadvantages, low molecular weight polyimides encapped with polymerizable groups such as ethylenically unsaturated groups, have been prepared. These end-capped polyimides can be subsequently cured to void-free, higher molecular weight resins since no volatile by-product is formed.
It is a purpose of this invention to provide novel linear, optionally crosslinked, polyimides which contain flexibilizing moieties and which may be, if desired, end-capped with polymerizable or inert groups. It is also an object to provide improved adhesives and molded articles. It is a further object to provide novel primary aromatic diamines for use in the preparation of the polyimides and the polyamic acid or ester intermediates.